In the past, thermal transfer ribbons have generally comprised a foil-like carrier, support or substrate, e.g. a paper or synthetic resin, a fusible color-transfer layer in the form of a wax-bonded dyestuff or pigment, and, in some cases, intervening layers between the substrate and the color transfer layer to facilitate such transfer if desired in the form of an image with the desired degree of resolution or definition.
When the term "color" is used herein, it will be understood to mean any dyestuff or pigment which is conventionally used in such color transfer layers and is intended to display the image of the heated pattern applied to the support foil by transfer to the colored layer or the coloring matter to the copy sheet. Since the term "color" includes black, as these expressions are used here, the pigment may be carbon black and the ribbon may be a thermal carbon ribbon.
The term "foil" as used herein is intended also to include support films and hence the ribbons involved may be of the type also known as film ribbons.
The ribbons generally pass between a thermal print head and the copy sheet. The thermal print head is provided with means adapted to apply heat and pressure in varying patterns to the support foil and to thereby cause melting of the transfer layer on the opposite surface of this foil and bonding of the material of the transfer layer to the copy sheet in the predetermined pattern. The print head may form a dot matrix printer and at each point or dot represented by a pin adapted to bear upon the ribbon, a corresponding dot is transferred to the copy sheet so as to form a printed symbol, e.g. an alphanumeric character. A thermal printer of the type with which the ribbon of the invention may be used is described, for example, in German open application DE-OS No. 3 224 445.
Customarily the thermal print head is pressed by a spring against the thermal transfer ribbon which is juxtaposed with a portion of the copy sheet lying against the platen of the printer, i.e. a roll which can be rotated to advance the copy sheet line-by-line relative to the print head.
The heating, in accordance with the alphanumeric character patterns, is effected at temperatures up to around 400.degree. C. to melt the transfer layer in accordance with the pattern developed by the print head and the molten material adheres to the paper or copy sheet to which it remains adherent as the copy sheet and the ribbon are relatively displaced. The used portion of the ribbon may be continuously taken up by a takeup spool.
Wax-bonded color-transfer layers are problematical to handle since the color can come off on the hands of the user or on surfaces which are contacted by the ribbon even in the absence of thermal activation temperatures. Furthermore, effective transfer of the color to the paper for good resolution and definition requires that the copy sheet or paper to which the transfer is made be relatively smooth.
Since it is also desirable to have good resolution and definition on somewhat rougher papers and to avoid the undesirable color transfer to the hands of the user and other objects, considerable research has been expended in developing an improved thermal transfer ribbon. Thus it has been found that the use of a synthetic resin or thermoplastic material as a binder for the color transfer layer can avoid the above-mentioned problems.
However, other problems have arisen when synthetic resin bonded color-transfer layers have been used. The color sharpness was relatively poor, for example.